The present invention relates to a method for making in-line optical waveguide refractive index gratings of any desired length and articles manufactured utilizing this method. More specifically, the present is directed to a method for making a pure-apodized, chirped fiber Bragg grating (FBG) of any length by translating a fiber with respect to an interferogram of actinic radiation with an intensity that is amplitude modulated as a function of time and to long-length continuous-phase Bragg gratings manufactured using this technique.
In-line optical waveguide refractive index gratings are periodic, aperiodic or pseudo-periodic variations in the refractive index of a waveguide. Gratings may be formed, for example, by physically impressing a modulation on the waveguide, by causing a variation of the refractive index along the waveguide using the photosensitivity phenomenon, or by other methods known in the art. In particular, gratings written into the core of an optical fiber are critical components for many applications in fiber-optic communication and sensor systems.
Dopants, such as germanium, are added to an area of the waveguide material to make it photosensitive, causing the refractive index of that region to be susceptible to increase upon exposure to actinic radiation. The currently preferred method of xe2x80x9cwritingxe2x80x9d an in-line grating comprises exposing a portion of the waveguide to the interference between two beams of actinic (typically UV) radiation. The two beams are incident on the guiding structure of the waveguide in a transverse direction to create an interferogram, that is, a pattern of optical interference. The angle between the two beams (and the wavelength of the radiation) defines the fringe spacing of the interferogram. Typically, the two beams of actinic radiation are the legs of an interferometer or are produced by launching a single beam through a phase mask. The phase mask method is considered generally more suitable for large scale manufacture of in-line gratings, because it is highly repeatable, less susceptible to mechanical vibrations of the optical setup, and can be made with writing beams of much shorter coherence length.
Advantages of optical fiber in-line gratings over competing technologies include all-fiber geometry, low insertion loss, high return loss or extinction, and potentially low cost. But one of the most distinguishing features of fiber gratings is the flexibility the gratings offer for achieving desired spectral characteristics. Numerous physical parameters of the gratings can be varied, including induced index change, length, apodization, period chirp, grating pitch tilt, and whether the grating supports coupling into co-propagating (long-period or transmission gratings) or counter-propagating coupling (Bragg gratings) at a desired wavelength. By varying these parameters, gratings can be tailored for specific applications.
The versatility of an in-line grating is largely dependent on two factors, the overall length of the grating structure and the reflectivity (or transmission) profile of the grating structure itself. Intricate reflectivity profiles can be achieved by carefully controlling the refractive index perturbation along the waveguide length, x. The index perturbation ∂n(x) may be characterized as a phase and amplitude-modulated periodic function,
∂n(x)=∂n0(x)xc2x7{A(x)+m(x)xc2x7cos[2xcfx80/xcex9xc2x7x+xcfx86(x)]},xe2x80x83xe2x80x83(1)
where ∂n0(x) is the xe2x80x9cdcxe2x80x9d index change spatially averaged over a grating period, A(x) is an offset (typically A=1), m(x) is the fringe visibility of the index change, xcex9 is the nominal period and xcfx86(x) describes grating chirp. To automate the fabrication process, it is desirable to write this arbitrary refractive index profile into a waveguide in a single process step, i.e., with a single pass of the laser beam over the waveguide and without physically changing the writing apparatus. For full flexibility in grating manufacture, one needs to control independently each of the parameters describing ∂n(x).
In particular, apodization of a grating spectrum may be achieved by controlling say ∂n0(x) and m(x) along the grating length. The main peak in the reflection spectrum of a finite length in-line grating with uniform modulation of the index of refraction is accompanied by a series of sidelobes at adjacent wavelengths. Lowering the reflectivity of the sidelobes, or xe2x80x9capodizingxe2x80x9d the reflection spectrum of the grating, is desirable in devices where high rejection of nonresonant light is required. Apodization also improves the dispersion compensation characteristics of chirped gratings. In most of these applications, one desires apodization created by keeping the average ∂n0(x) and A(x) constant across the grating length while m(x) is varied, which is believed not to have been achieved (with full flexibility) in a single-step process by controlling only the laser beam.
Variation of the index modulation by changing the magnitude of the ultraviolet exposure along the length of the grating causes both the magnitude of the refractive index modulation and the average photoinduced refractive index to vary. The variation in the average index modulation leads to undesirable effective chirps of the resonant wavelength of the grating and widens the grating spectral response. To alleviate these symptoms, it is desirable to xe2x80x9cpure apodizexe2x80x9d the grating, that is, to generate both the non-uniform modulated ultraviolet fringe pattern and a compensating DC exposure which automatically ensures that the average photoinduced refractive index is constant along the length of the fiber.
Some researchers have created the desired apodization profile by dithering the phasemask relative to the interferogram. The dithering decreases the fringe visibility and thus the refractive index modulation at specified locations along the waveguide length. However, the technique requires complex mechanical fixtures that must be vibrated yet precisely positioned for the phase mask and waveguide.
In addition to the specific index perturbation written into the waveguide, grating length is also important in certain applications in optical fiber communication and distributed sensor systems. For instance, long-length chirped fiber Bragg gratings have been suggested as attractive devices for the manufacture of dispersion compensators. High-speed, long distance data transmissions, especially transmissions over existing non-dispersion shifted fiber networks, are limited by chromatic dispersion in the optical fiber. Since the transmission bandwidth usually is predetermined by the needs of the system, to be usable as dispersion compensators in practice, chirped Bragg gratings need to exhibit dispersion compensation over a bandwidth which will cover the typical semiconductor laser wavelength tolerances. However such narrow band devices may result in unusable wavelengths in the regions where the FBG band edges occur, thus large bandwidth chirped FBGs which can to compensate over the full Er+-doped fiber amplifier spectrum are more desirable.
Presently, most telecommunications systems possess an installed base of fiber which is dispersion corrected for 1300 nm transmission but, not for 1550 nm transmission. With the availability of Er-doped fiber amplifiers at 1550 nm and the low loss limit of the fiber occurring in the same wavelength range, high bit rate transmission systems have migrated to the 1550 mn wavelength range. Fiber dispersion at 1550 nm for these nondispersion shifted fibers is near 17 ps/nm/km. Over an 80 km distance this results in roughly xe2x88x921360 ps/nm of excess dispersion, which requires correction before optical pulses can be detected. Dispersion compensating fiber is a preferred choice for the correction of chromatic dispersion in this wavelength range. While being broad band fiber nonlinearities and high loss are drawbacks to this technology. A long-length phase- continuous fiber grating which will compensate large bandwidths for chromatic dispersion may be a desirable alternative to the fiber solution.
Phase-continuous fiber Bragg gratings used in dispersion compensation typically possesses 0.5, 1.5 and 7 nm bandwidths when being used to correct for 1360 ps/nm of excess dispersion. Narrow band gratings of 0.5 and 1.5 nm are roughly 10 to 30 cm in length and are typically fabricated using e-beam written phase masks. Long-length broadband chirped gratings are usually fabricated using some type of phase mask/fiber scanning technique. Ideally a desired grating based dispersion compensator will cover the entire Erbium doped fiber amplifier bandwidth with the appropriate compensation for the dependence of the dispersion on wavelength. Such gratings would have bandwidths in excess of 40 nm and would necessitate lengths  greater than 800 cm to compensate for an 80 km link length. Longer link lengths will require even longer gratings. For example a 120 km transmission span would require a 40 nm bandwidth fiber grating  greater than 1200 cm in length. Grating based dispersion compensators have the added advantage of smaller package size than fiber, low nonlinearities at high input powers and easily definable delay response with wavelength.
Presently, the length of gratings imprinted using traditional methods and phase masks is limited by the length of available phase masks, to about ten to fifteen centimeters. The longest reported gratings, even when manufactured under exacting conditions, have been limited to  less than 2.5 meters. The need exists for accurate longer length Bragg gratings having complex grating structures, which may be manufactured in a cost-effective manner.
One method has been described where a UV-beam is scanned over a 10 to 15 cm long phase mask having a fixed position relative to the fiber. Complex structures are added by varying the exposure time or by postprocessing the grating. Another method discusses the use of fibers held in a fixed position relative to specially designed long phase masks having the complex structure already imprinted in the mask. However, as indicated above, the length of available phase masks limits both of these techniques.
Some attempts at long length devices have been made by translating a fiber with high-precision staging relative to an interferogram of UV-light. The position of the stage is trackedinterferometrically, and the laser is triggered when the fiber reaches the desired position as determined by feed back from the stage for the next irradiation. The phasing between these subgratings can be controlled to create some complex structures, such as chirps, and apodization can be achieved by dithering about an interferogram/fiber relative position. Using grating stitching methods, groups at Nortel, The Royal Institute of Technology (Sweden), and the University of Southamptom (U.K.) have reported FBGs of lengths longer than a phase mask. The longest length FBGs reported have been  less than 2.5 m in length, which is not long enough to compensate for dispersion over the full Er+amplifier wavelength band for an 80 km link length of nondispersion shifted fiber. These methods of stitching subgratings together are limited, since the length of a grating is restricted to the length of motion of available precision translation stages, which are, at most, a couple meters in length and require interferometer feed back.
Recent developments have attempted to produce long complex gratings by scanning a UV-beam over a phase mask and writing sub-gratings (a number of grating elements), which are then made near phase continuous by UV-trimming between each subgrating. To increase the size of the grating structure, a number of subgratings may then be concatenated to one another. UV-trimming is applied between the gratings to make a seemingly continuous-phase grating of tens of centimeters. The fiber is translated with high-precision staging relative to an interferogram of UV-light. The position of the stage is tracked interferometrically and the laser is triggered when the fiber reaches the desired position for the next subgrating. The phasing between these subgratings may be controlled to create some complex structures, such as chirps. Apodization may be achieved by dithering about an interferogram/fiber relative position.
The concatenation process suffers from needing extremely accurate positioning staging, which is currently available only by using an interferometer as an encoder. The resulting gratings frequently are of poor quality and are extremely difficult and time-consuming to make. The maximum practical length achievable using this method is only a few tens of centimeters. Presently only linear motion staging can be interoferometrically controlled; rotary stages must use ruled encoders. Therefore, the length of a fiber grating made with a concatenation process is limited by the linear travel available on precision stages and the acceptable writing error.
Another problem with present methods is that since the protective housing around a fiber must be removed for grating fabrication, a long length of bare fiber containing the grating is removed from the precision staging and coiled for packaging. The longer the length of bare fiber used, the more fabrication complexity increases (increased handling), which complicates manufacture automation and is likely to reduce the mechanical strength of the fiber.
The need remains for increasingly longer gratings to satisfy present and future optical applications and for an accurate, cost-effective writing technique for very long length in-line optical waveguide gratings having complicated reflectivity profiles.
The present invention discloses a novel method for manufacturing a grating of any length, with independent control of each parameterdefining the index perturbation. The length of the gratings achieved by the present invention is not limited either by the size of the phase mask or the need for a long-length linear motion stage requiring an inteferometer providing xcx9c1 nm resolution. The present specification describes continuous phase Bragg reflectors achieved using a rotary stage having novel characteristics, including previously unattained length dimensions.
In a method of manufacturing in accordance with the present invention, a photosensitive waveguide, such as an optical fiber, is provided. A writing beam of actinic radiation, such as a UV laser beam, is positioned to write on the fiber. A periodic intensity distribution is obtained, for example by using an interference pattern generator such as a phase mask positioned between the writing beam and the waveguide to create an interferogram of period xcex9.
The waveguide then is translated through the periodic intensity distribution relative to the writing beam at a precisely-controlled relative velocity v(t). Alternatively, for applications requiring long-length gratings (but not limited to long-length FBGs), the fiber may be coupled to a spool which rotates to draw the fiber at v(t) through the periodic intensity distribution. Finally, a modulator varies the amplitude of the beam intensity as a function of time at a frequency f(t) such that v(t)/f(t)≈xcex9.
The writing beam at the fiber has a peak intensity I0 and a width D. The fluence xcfx86(x) delivered to the fiber is determined by the equation                               Φ          ⁡                      (            x            )                          ≈                                            I              0                        4                    ·                      D                          v                              (                x                )                                              ·                      {                          1              -                                                1                  2                                ·                                  cos                  ⁡                                      [                                                                                            ω                                                      (                            x                            )                                                                                                    v                                                      (                            x                            )                                                                                              ·                      x                                        ]                                                                        }                                              (        2        )            
where xcfx89=2xcfx80xc2x7f. Either v or xcfx89 may be kept constant during the writing process. Either parameter may be detuned to chirp the refractive index perturbation along the grating length x=vxc2x7t.
The method also may include the step of controlling further the intensity of the writing beam to vary the visibility of the index variation, m, and peak intensity illuminating the fiber, I0. The offset of the oscillating index perturbation, A, also may be controlled. The flux delivered to the fiber is then determined by the equation                               Φ          ⁡                      (            x            )                          ≈                                            I              0                              (                x                )                                      4                    ·                      D                          v                              (                x                )                                              ·                      {                                          A                                  (                  x                  )                                            -                                                                    m                                          (                      x                      )                                                        2                                ·                                  cos                  ⁡                                      [                                                                                            ω                                                      (                            x                            )                                                                                                    v                                                      (                            x                            )                                                                                              ·                      x                                        ]                                                                        }                                              (        3        )            
Modulation of the index variation visibility, m, allows the fabrication of pure-apodized gratings. By varying these additional parameters, i.e., the amplitude and offset of the refractive index oscillations, the refractive index envelope along the fiber length can be precisely controlled.